Direct chip attachment (DCA) with electrically conductive adhesives

ABSTRACT

A simple process for card assembly by Direct Chip Attachment (DCA) uses electrically conductive adhesives. Two methods create the same intermediate wafer product with a layer of insulative thermoplastic and conductive thermoplastic bumps. After sawing or dicing the wafer to form the chips, the chips are adhered to chip carriers with conductive pads which match the conductive thermoplastic bumps, using heat and pressure. Chips may be easily removed and replaced using heat.

This application is a continuation of application Ser. No. 08/190,507,filed on Feb. 2, 1994 now U.S. Pat. No. 5,543,585.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the manufacture of electroniccircuits and, more particularly, to a module structure and a simpleprocess for card assembly by Direct Chip Attachment (DCA), usingelectrically conductive adhesives, which are reworkable.

1. Background

An involved card assembly attach process exists for Direct ChipAttachment (DCA). Unique decals are fabricated with a plated solderpattern corresponding to the chip pad footprint. These decals areaccurately positioned onto the intended footprint of the chip carrierand the solder is subsequently transferred to the carrier sites in areflow oven. The soldered pads are then leveled mechanically forcoplanarity and finally the chips are placed and reflowed.

Solder alloys are used pervasively to interconnect components tocarriers. Undesirable attributes of solder alloys include hightemperature joining, cleaning/residues, fatigue life and theenvironmental ramification of lead (Pb).

Moore et al., U.S. Pat. No. 5,120,678, disclose a method for assemblingan integrated circuit component to a substrate by a solder byinterconnection that is reinforced by a polymer film. While this methodencapsulates ball grid arrays with a thermoset adhesive, the thermosetis not reworkable.

An alternative to solder DCA is electrically conductive adhesivejoining. Screen printing 0.004" features on 0.008" centers is possiblewith some electrically conductive adhesives.

U.S. Pat. Nos. 5,074,947 to Estes and 5,086,558 to Grube, discloseattaching flip chips to substrates using conductive polymer materials.All the above cited patents are incorporated herein by reference.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processto attach a flip-chip to a substrate using a thermoplastic,isotropically conducting adhesive.

According to a preferred embodiment of the invention, there is provideda two pass screen process in which a thermoplastic adhesive is appliedto the pad side of the wafer. This two pass process can be accomplishedin either of two methods where, in one pass, an electrically conductivethermoplastic adhesive is applied to the chip bond pads. A second passapplies an electrically insulating thermoplastic adhesive to thenon-conductive area of the same wafer surface. The methods differprimarily by the order of the passes.

In a second preferred embodiment of this invention, individual chips areheated to a temperature at which the thermoplastic adhesive is in atacky state, in preparation for bonding to a substrate. Placement andbonding is accomplished in one step. The electrical pads on the chip andsubstrate are interconnected with the electrically conductive adhesive.

It is further an object of the present invention to provide a method ofpreparing a wafer for DCA bonding with electrically conductive adhesivesusing current art in adhesive application.

The assembly process is a one step process which accomplishes placement,joining and encapsulation and which is reworkable. The processeliminates the need for many of the commonly required steps to preparechip sites on the substrate for DCA. The process also eliminates theneed for solder reflows, two flux application, post attach cleaning,application and cure of thermoset encapsulant.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1, 2 and 3 show cross-sectional views of a substrate in theprocess of adding insulative and conducting thermoplastic to thesubstrate, according to the first method of the invention;

FIG. 4 is a flow diagram showing the process steps of the first method;

FIGS. 5, 6 and 7 show cross-sectional views of a substrate in theprocess of adding insulative and conducting thermoplastic to thesubstrate, according to the second method of the invention;

FIG. 8 is a flow diagram showing the process steps of the second method;

FIG. 9 is a cross-sectional view of a finished substrate with insulativeand conducting thermoplastic;

FIG. 10 is a wafer with a thermoplastic layer ready for sawing or dicinginto chips;

FIG. 11 is a flow diagram showing the process of creating chips byeither of the two methods and adding them to a chip carrier;

FIG. 12 is a plan view of die for assembling chips on a chip carrier;

FIG. 13 is a plan view of a chip carrier;

FIG. 14 is a cross-sectional view of a chip adhered to a chip carrier;and

FIG. 15 is a flow diagram of reworking a chip and chip carrier.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown a substrate 1. The substrate may be a wafer or chip withintegrated circuits or a substrate (ceramic, organic, or flexible) towhich an integrated circuit chip is to be attached, such as a chipcarrier module or a circuit board/card. In a specific embodiment, awafer, prior to the practice of the first method according to theinvention, has a plurality of chip sites at which integrated circuitshave been formed. Each chip site includes a matrix of thin conductivemetal contacts 8. After the processing according to the invention, thewafer is sawn or diced into the individual chips. In FIG. 2, aninsulative thermoplastic adhesive 2 is added to the surface of substrate1 where the contact pads 8 are located. After baking or heating theinsulative adhesive 2, holes 3 are made in the adhesive layer 4, asshown in FIG. 3. The holes 3 may be made in the insulative adhesivelayer 2 at each metallic pad by various methods including:

1. spin coating onto the substrate 1, drying, and then, using laseroblation of the adhesive material in pad areas 8 to make the holes

2. bonding a dry insulative adhesive film which is prepunched with holes3 directly to the substrate 1 (a stiff backing may be required duringpunching), or

3. screen printing to place insulative adhesive everywhere on thesubstrate surface except chip bond pads 8 and then drying. The holes 3are then filled with conductive thermoplastic adhesive and baked orheated at an elevated temperature such as 80°-150° C. and preferably atabout 120° C. to dry the conductive adhesive.

FIG. 4 shows a flow diagram of the first method. The first step 31 is tocoat the substrate with the insulative adhesive, and after curing, holesare cut in the insulative layer in step 32. Finally, the holes are filedwith a conductive adhesive in step 33, and the conductive adhesive isdried.

FIGS. 5 through 7 illustrate the second method according to theinvention. FIG. 5 shows a cross-sectional view of the substrate 1 withconductive metal pads 8, prior to processing. In FIG. 6, conductiveadhesive bumps or pegs 4 are added to the substrate 1. Preferably, thebumps are applied by mask screening. The conductive thermoplastic mayalso be added to a matrix of conductive metal pads 8 on anothersubstrate to which the first substrate is to be attached to assist inbonding. The conductive thermoplastic is then dried. Insulative adhesive2 is applied to the wafer between conductive adhesive 4 bumps or pegs tocreate the composite structure 5 which covers the entire surface of thesubstrate, as shown in FIG. 7. Prior to adding insulative adhesivepaste, the conductive bumps may be plated (electroless, electroplate, orevaporation) with a very thin layer of low melting temperature metalsuch as Ag to prevent interaction with the insulative adhesive (e.g.dissolving the pegs). This product is baked to dry the insulativethermoplastic.

Prior to applying the insulating adhesive, there may be a layer ofsolder resist (procoat) on the substrate such as PSR-4000 or VACREL.Also, there may be a passivation layer on the chip such as polyimide ornitride. The insulative adhesive must be compatible with these chip andcarrier surfaces in order to reliably bond the chip to the carriersubstrate with the single layer of adhesive.

In this second method, the insulative adhesive may be applied by bondinga prepunched film, screen printing, or spreading a flowable paste oremulsion (e.g. spin coating) directly onto the substrate surface.

FIG. 8 shows a flow diagram of the second method. The first step 81 isto add pegs of conductive thermoplastic adhesive to chip pad sites onthe wafer. Then, in step 82, the insulative layer of adhesive isprovided around the pegs.

FIG. 9 is a cross-sectional view of the final product, whether producedby the first or second methods. As can be seen in this view, theconductive adhesive 4 is surrounded by the insulative adhesive 2, andthe composite adhesive structure 5 covers the entire surface of thesubstrate. At this stage, the conductive adhesive may project up abovethe level of the surrounding insulative adhesive and the holes may havea larger diameter than the bumps or pegs of conductive adhesive, but thevolume of the conductive adhesive should be about the same as the holes,so that upon attachment, the two adhesives completely fill the spacebetween the chip and substrate. FIG. 10 illustrates a wafer 1, with acomposite thermoplastic layer 5 ready to be sawn or diced into chips 6.The thickness of the thermoplastic adhesives must be sufficient tocompensate for any bowing of the substrates (such as an FR-4 circuitboard), and metal pad height to 1 arances, and to form a good void freebond. Preferably, the thickness is 1-5 mils for ceramic and flexiblesubstrates and 3-8 mil to attach chips to organic substrates, such asFR-4.

As can be understood from the foregoing, chips in the wafer are formedwith an electrically conductive adhesive in a two pass screen process toapply adhesive to the pad side of the wafer. The order of the steps isnot controlling. The first pass applies either the insulative orconductive adhesive to the chip bond pads. The second screening wouldapply the same or similar adhesive paste with a conductive or insulativefiller to the same substrate surface. The addition of insulative fillerto the insulative adhesive is optional depending on coefficient ofthermal expansion (CTE) requirements. After each screening, solventwould be driven off in a bake or vacuum/bake step. The completed waferis then ready for dicing.

The insulative thermoplastic adhesive is a dry fully polymerizeddielectric adhesive preferably selected from one of the followingpolymer systems and may be filled with dielectric solids preferablyceramic particles, quartz, or glass. Preferred dielectric polymersystems include thermoplastic polymers, copolymers, or blends. Examplesof polymers include, but are not limited to, nylons, polysulfones,polyesters, flexible and soluble polyimides and siloxanes. Copolymersinclude, but are not limited to, random, segmented or block copolymersmade of ethylene and vinyl acetate (EVA), siloxane and aryl-ethers,polyurethanes composed of polyisocyanate and polyethers or polyestersegments. Blends may be of two or more polymers/copolymers such asethylene-acrylate copolymer with polyethylene, EVA and poly(vinylchloride) (PVC), polyester and PVC. Conductive thermoplastic systems arecomposed of fully polymerized thermoplastic systems such as the aboveand electrically conductive particles made of or coated by Au, Ag, Pd ortheir oxide free, noble alloys. In order to form a paste for applicationthe fully polymerized thermoplastic system and fillers are thinned witha solvent or mixed a liquid to form an emulsion. After application thematerial is heated at an elevated temperature to evaporate the solvantor liquid until dry (non-tacky). In order to bonds the chip to thesubstrate the adhesive must be heated significantly above the transitiontemperature until it becomes tacky. The insulating and conductiveadhesive materials are preferably selected to be compatible duringattachment processing. The thermoplastic materials must be chemicallycompatible so they do not negatively affect each others properties andpreferably so that they bond together tightly for strength. Also,preferably the properties of the thermoplastics are similar to provideabout the same CTE to minimize stresses and provide overlappingpreferred drying and bonding temperature ranges.

FIG. 11 is a flow diagram illustrating preparing a chip to be placed ona chip carrier, starting with a wafer. As can be seen in the diagram,the wafer can be prepared by either method, here indicated as Method Iand denoted by reference numeral 111, where insulative thermoplasticadhesive paste is added to the wafer, dried (unless added as apre-punched dry film adhesive bonded directly to the wafer surface), andthen conductive thermoplastic adhesive paste is added to holes made inthe insulative adhesive, or Method II and denoted by reference numeral112, where conductive pegs of adhesive paste are added to the wafer,dried, then the insulative thermoplastic paste is applied to fill gapsbetween the conductive thermoplastic pegs. Following either Method I orMethod II, the wafer is diced into chips in step 113. The chips areplaced in a die in step 114, and the die is pressed against a chipcarrier and heat is added in step 115. When the die is removed in step116, the chip remains adhered to the chip carrier. An optional step inthis process is to add conductive thermoplastic adhesive to the pads onthe chip carrier and drying the optional adhesive before pressing thedie and chips against the chip carrier.

Alternative manufacturing methods not illustrated include, applying theinsulative adhesive to the chip carrier and the conductive adhesivebumps to the chip. After fitting the matching pieces together, heat andpressure are applied as in other methods. Also, both the insulative andconductive adhesives may be applied to the chip carrier by Methods I andII, using heat and pressure to attach the chip to the prepared chipcarrier. Furthermore, the insulative and conductive adhesives may beapplied to the chip carrier and in addition, conductive adhesive may beapplied to the metal contact pads on the chip.

FIG. 12 illustrates a die 10 which may be used to add chips to a chipcarrier 12, illustrated in FIG. 13. Chip carrier 12 will have conductivepads to match the bumps of conductive adhesive 4 in the chip 6 to beadhered to the chip carrier. The chips 6 are placed in recesses 13 onthe die. The die 10 is then pressed against the chip carrier 12 and heatis added. When the die is removed, the chips remain adhered to the chipcarrier. FIG. 14 shows a single chip adhered to the printed chip carrierafter assembly. Here, a chip 14 is adhered to a chip carrier 12 with alayer of insulative thermoplastic adhesive 2 filling gaps betweenconductive thermoplastic adhesive 4 bumps. The electrically insulatingthermoplastic adhesive completely fills the volume between the chip andthe chip attach site around the electrically conductive adhesive pegswhich adhesively connect between confronting pairs of metal contactpads.

This process simplifies mass production of chips mounted to carriers byremoving the need for solder and makes it possible to replace chipssince the thermoplastic adhesive is reworkable. Reworking is illustratedin the flow chart depicted in FIG. 15. Because this is a thermoplasticadhesive, reworking is relatively simple and inexpensive. To replace asingle chip in a chip carrier, a die to fit the single chip is placedover the chip in step 151. The die is heated and then removed, takingthe chip with it in step 152. Any remaining adhesive may require removalusing known solvents depending on the adhesive system. A new chip isthen placed in a single chip die, pressed against the board with heat,and adhered to the board, much the same as in the original assembly ofthe printed chip carrier. This is a particularly useful feature of thisinvention which is not found in other methods of chip to carrierassembly. In other chip attachment technologies, if a single chip of achip carrier is bad, in most cases the whole must be replaced. In othercases, there is an involved process of filing thermosetting polymersremaining on the chip carrier after removing the bad chip. The processaccording to the invention makes chip replacement a simple andeconomically viable process.

A step which may be optional depending on chip size and planarity of thecarrier surface would be to screen dispense electrically conductiveadhesive onto the carrier pads as well and to dry this adhesive prior toconnection (heat bonding).

In preparation for bonding chips to a chip carrier or circuit board,individual chips are heated to a temperature significantly above thetransition temperature at which the thermoplastic adhesive is in a soft,tacky state. The surface of the carrier chip site is heated similarly. Atypical temperature would be 150° to 300° C. An automatic chip placementtool would be used to pick the chip and place the chip on the chip siteof the carrier. Placement and bonding is accomplished in one step.Bonding occurs with time at temperature and pressure. The placement toolcan be set to apply the required bonding load (e.g.>30 psi) for thenecessary time (e.g. several seconds). The electrical pads of the chipand carrier are interconnected with the electrically conductiveadhesive. The insulative thermoplastic adhesive mechanically joins thechip to the carrier surface.

While the invention has been described in terms of two preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by letters patent is as follows:
 1. A method of making integratedcircuit chips comprising the steps of:applying an insulativethermoplastic adhesive paste, over a semiconductor wafer substratehaving a plurality of chip sites and integrated circuits communicatingwith a matrix of conductive metal pads on a major surface of thesubstrate; drying the insulative thermoplastic; forming holes in theinsulative thermoplastic layer at the metal pads; filling the holes witha conductive thermoplastic adhesive paste; and drying the conductivethermoplastic to form a dry composite adhesive layer dividing the waferinto a plurality of chips after forming the composite layer.
 2. Themethod recited in claim 1 wherein the step of applying an insulativeadhesive is performed by spin coating the substrate surface with aadhesive paste and drying, and the step of forming holes in theinsulative thermoplastic layer is performed by laser ablation.
 3. Themethod recited in claim 1 wherein the insulative adhesive layer is driedand pre-formed with punched holes and then bonded directly to the chipsurface with heat and pressure.
 4. The method recited in claim 1 whereinthe insulative adhesive layer is applied by emulsion screening to placethe adhesive everywhere except chip bond pads, thereby simultaneouslyperforming the steps of applying the adhesive and forming the holes. 5.The method recited in claim 1 wherein the substrate is an integratedcircuit wafer having a plurality of chip sites, and further includingthe step of dividing the wafer into a plurality of chips after dryingboth thermoplastic adhesives.
 6. The method recited in claim 1 whereinthe drying steps include heating to a temperature of at least about 80°C.
 7. The method recited in claim 1 further including the step ofheating the substrate to a temperature between about 150° C. and about300° C. to bond the adhesives to the substrate.
 8. The method recited inclaim 1 wherein the holes are filled with conductive adhesive paste bymask screening.
 9. A method of making integrated circuit chipscomprising the steps of:providing a semiconductor wafer substrate havinga plurality of chip sites and integrated circuits communicating with amatrix of conductive metal contact pads on a major surface of thesubstrate; depositing conductive thermoplastic adhesive paste bumps onthe metal pads; drying the conductive adhesive; applying an insulativethermoplastic adhesive over the substrate surface between the conductiveadhesive deposits; curing the insulative adhesive to form a drycomposite adhesive layer; and dividing the wafer into a plurality ofchips after forming the composite layer.
 10. The method recited in claim9 wherein the step of applying the insulative adhesive is performed byemulsion screening of adhesive paste.
 11. The method recited in claim 9wherein the step of applying the insulative adhesive is performed byforming the insulative adhesive into a film, dried, punched and bondeddirectly to the chip surface by heat and pressure.
 12. The methodrecited in claim 9 wherein the step of applying the insulative adhesiveis performed by flowing adhesive paste over spaces between theconductive thermoplastic bumps.
 13. The method recited in claim 9further comprising the step of plating the conductive adhesive bumpswith a layer of conductive metal prior to applying an insulativeadhesive paste.
 14. A method of preparing substrates for attachmentcomprising the steps of:providing a first semiconductor wafer substratehaving a plurality of chip sites with a matrix of conductive metal padson a major surface; depositing an electrically conductive thermoplasticadhesive paste on the metal pads; depositing an electrically insulatingthermoplastic adhesive on the substrate surface around the metal pads;drying the paste; and dividing the semiconductor wafer into a pluralityof individual chips after said depositing steps and said drying step.15. The method recited in claim 14 wherein the electrically insulatingadhesive is deposited on the substrate as a paste followed by theadditional step of drying the insulating adhesive prior to the step ofdepositing the conductive adhesive on the pads.